Conductive silver paste for a metal-wrap-through silicon solar cell

ABSTRACT

A conductive silver via paste comprising particulate conductive silver, a vanadium-phosphorus-antimony-zinc-based-oxide, a tellurium-boron-phosphorus-based-oxide or a tellurium-molybdenum-cerium-based-oxide and an organic vehicle is particularly useful in providing the metallization of the holes in the silicon wafers of MWT solar cells. The result is a metallic electrically conductive via between the collector lines on the front side and the emitter electrode on the back-side of the solar cell. The paste can also be used to form the collector lines on the front-side of the solar cell and the emitter electrode on the back-side of the solar cell. Also disclosed are metal-wrap-through silicon solar cells comprising the fired conductive silver paste.

FIELD OF THE INVENTION

This invention is directed to a conductive silver paste for use in ametal-wrap-through (MWT) silicon solar cell and to the respective MWTsilicon solar cells made with the conductive silver paste.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional solar cell with a p-type (p-doped) silicon base has ann-type (n-doped) emitter in the form of an n-type diffusion layer on itsfront-side. This conventional silicon solar cell structure uses anegative electrode to contact the front-side, i.e. the sun side, of thecell and a positive electrode on the back-side. It is well known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor serves as a source of external energy to generateelectron-hole pairs. The potential difference that exists at a p-njunction causes holes and electrons to move across the junction inopposite directions, thereby giving rise to flow of an electric currentthat is capable of delivering power to an external circuit. Most solarcells are in the form of a silicon wafer that has been metallized, i.e.,provided with metal electrodes which are electrically conductive.Typically, the front-side metallization is in the form of a so-called Hpattern, i.e. in the form of a grid cathode comprising thin parallelfinger lines (collector lines) and busbars intersecting the finger linesat right angles, whereas the back-side metallization is an aluminumanode in electric connection with silver or silver/aluminum busbars ortabs. The photoelectric current is collected by means of these twoelectrodes.

Alternatively, a reverse solar cell structure with an n-type siliconbase is also known. This cell has a front p-type silicon surface (frontp-type emitter) with a positive electrode on the front-side and anegative electrode to contact the back-side of the cell. Solar cellswith n-type silicon bases (n-type silicon solar cells) can in theoryproduce higher efficiency gains compared to solar cells with p-typesilicon bases owing to the reduced recombination velocity of electronsin the n-doped silicon.

As in the case of the conventional silicon solar cells, MWT siliconsolar cells can be produced as MWT silicon solar cells having a p-typesilicon base or, in the alternative, as MWT silicon solar cells havingan n-type silicon base. As in conventional solar cells, the emitter of aMWT solar cell is typically covered with a dielectric passivation layerwhich serves as an antireflective coating (ARC) layer. However, MWTsilicon solar cells have a cell design different than that of theconventional solar cells. The front-side electrodes of conventionalsolar cells reduce the effective photosensitive area available on thefront-side of the solar cell and thereby reduce performance of the solarcell. MWT solar cells have both electrodes on the back-side of the solarcell. This is accomplished by drilling, e.g., with a laser, small holesthat form vias between the front-side and the back-side of the cell.

The front-side of the MWT silicon solar cell is provided with afront-side metallization in the form of thin conductive metal collectorlines which are arranged in a pattern typical for MWT silicon solarcells, e.g., in a grid- or web-like pattern or as thin parallel fingerlines. The collector lines are applied from a conductive metal pastehaving fire-through capability. After drying, the collector lines arefired through the front-side dielectric passivation layer thus makingcontact with the front surface of the silicon substrate. The term “metalpaste having fire-through capability” means a metal paste which etchesand penetrates through (fires through) a passivation or ARC layer duringfiring thus making electrical contact with the surface of the siliconsubstrate.

The inside of the holes and, if present, the narrow rim around thefront-edges of the holes, i.e., the diffusion layer not covered with thedielectric passivation layer, is provided with a metallization either inthe form of a conductive metal layer on the sides of the hole or in theform of a conductive metal plug that completely fills the hole withconductive metal. The terminals of the collector lines overlap with themetallizations of the holes and are thus electrically connectedtherewith. The collector lines are applied from a conductive metal pastehaving fire-through capability. The metallizations of the holes aretypically applied from a conductive metal paste and then fired. Themetallizations of the holes serve as emitter contacts and form back-sideelectrodes connected to the emitter or electrically contact other metaldeposits which serve as the back-side electrodes connected to theemitter.

The back-side of a MWT silicon solar cell also has the electrodesdirectly connected to the silicon base. These electrodes areelectrically insulated from the metallizations of the holes and theemitter electrodes. The photoelectric current of the MWT silicon solarcell flows through these two different back-side electrodes, i.e., thoseconnected to the emitter and those connected to the base.

Firing is typically carried out in a belt furnace for a period ofseveral minutes to tens of minutes with the wafer reaching a peaktemperature in the range of 550° C. to 900° C.

The efficiency of the MWT solar cells is improved since the emitterelectrode is located on the back-side and thereby reduces shadowing ofthe photosensitive area available on the front-side of the solar cell.In addition the emitter electrodes can be larger in size and therebyreduce ohmic losses and all electrical connections are made on theback-side.

When producing a MWT solar cell there is a need for a conductive pastethat results in a metalized hole that: (1) has sufficiently low seriesresistance between the collector lines and the emitter electrode, (2)has good adhesion to the sides of the hole and to the silicon on thebackside of the solar cell and (3) has sufficiently high shuntingresistance to prevent deleterious electrical connection between portionsof the cell, i.e., the emitter and the base.

SUMMARY OF THE INVENTION

The present invention relates to conductive silver paste comprising:

-   -   (a) silver;    -   (b) a vanadium-phosphorus-antimony-zinc-based-oxide comprising        45-60 wt % V₂O₅, 15-30 wt % P₂O₅, 5-20 wt % Sb₂O₃ and 3-15 wt %        ZnO, wherein the wt % are based on the total weight of the        vanadium-phosphorus-antimony-zinc-based-oxide; and    -   (c) an organic vehicle, wherein the silver and the        vanadium-phosphorus-antimony-zinc-based-oxide are dispersed in        the organic vehicle.

The invention also relates to conductive silver paste comprising:

-   -   (a) silver;    -   (b) a tellurium-boron-phosphorus-based-oxide comprising 80-95 wt        % TeO₂, 1-10 wt % B₂O₃ and 1-10 wt % P₂O₅, wherein the wt % are        based on the total weight of the        tellurium-boron-phosphorus-based-oxide; and    -   (c) an organic vehicle, wherein the silver and the        tellurium-boron-phosphorus-based-oxide are dispersed in the        organic vehicle.

The invention further relates to conductive silver paste comprising:

-   -   (a) silver;    -   (b) a tellurium-molybdenum-cerium-based-oxide comprising 45-65        wt % TeO₂, 20-35 wt % MoO₃ and 10-25 wt % CeO₂, wherein the wt %        are based on the total weight of the        tellurium-molybdenum-cerium-based-oxide; and    -   (c) an organic vehicle, wherein the silver and the        tellurium-molybdenum-cerium-based-oxide are dispersed in the        organic vehicle.

These conductive silver pastes are particularly useful in providing themetallization of the holes in the silicon wafers of MWT solar cells.This metallization results in a metallic electrically conductive viabetween the collector lines on the front side and the emitter electrodeon the back-side of the solar cell.

Also provided is a metal-wrap-through silicon solar cell comprising thefired conductive silver pastes of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The conductive silver via pastes of the present invention allow for theproduction of MWT silicon solar cells with improved performance. Theconductive silver pastes have good hole filling capability. The resultis a metallic electrically conductive via between the collector lines onthe front side and the emitter electrode on the back-side of the solarcell. The paste can also be used to form the collector lines on thefront-side of the solar cell and the emitter electrode on the back-sideof the solar cell

The conductive silver pastes comprise silver, avanadium-phosphorus-antimony-zinc-based-oxide, atellurium-boron-phosphorus-based-oxide or atellurium-molybdenum-cerium-based-oxide, and an organic vehicle.

Each constituent of the conductive silver paste of the present inventionis discussed in detail below.

Silver

In the present invention, the conductive phase of the paste is silver(Ag). The silver can be in the form of silver metal, alloys of silver,or mixtures thereof. Typically, in a silver powder, the silver particlesare in a flake form, a spherical form, a granular form, a crystallineform, other irregular forms and mixtures thereof. The silver can beprovided in a colloidal suspension. The silver can also be in the formof silver oxide (Ag₂O), silver salts such as AgCl, AgNO₃, AgOOCCH₃(silver acetate), AgOOCF₃ (silver trifluoroacetate), silverorthophosphate (Ag₃PO₄), or mixtures thereof. Other forms of silvercompatible with the other thick-film paste components can also be used.

In one embodiment the silver is in the form of spherical silverparticles. The powder of spherical silver particles has a relativelynarrow particle size distribution. In an embodiment the spherical silverparticles have a d₅₀ of from 1.7 to 1.9 μm, wherein the median particlediameter, d₅₀, is determined by means of laser diffraction. In one suchembodiment, the spherical silver particles have a d₁₀≧1 μm and a d₉₀≦3.8μm. In another embodiment, the silver is in the form of irregular(nodular) silver particles having a d₅₀ of from 5.4 to 11.0 μm. and ad₉₀ of 9.6-21.7 μm The d₁₀, d₅₀ and d₉₀ represent the 10th percentile,the median or 50th percentile and the 90th percentile of the particlesize distribution, respectively, as measured by volume. That is, the d₅₀(d₁₀, d₉₀) is a value on the distribution such that 50% (10%, 90%) ofthe particles have a volume of this value or less.

The silver may be uncoated or the surface at least partially coated witha surfactant. The surfactant may be selected from, but is not limitedto, stearic acid, palmitic acid, lauric acid, oleic acid, capric acid,myristic acid and linolic acid and salts thereof, e.g., ammonium, sodiumor potassium salts. In one embodiment the surfactant is diethyleneglycol and the particle surfaces are essentially completely coated.

In another embodiment, the silver is in the form of silver flake. In oneembodiment, an average particle size of the silver flake is less than 10microns. In another embodiment, the average particle size is less than 5microns.

The silver is present in the conductive silver paste in a proportion of85 to 95 wt %, based on the total weight of the conductive silver paste.In one embodiment, the silver is present in the conductive silver pastein a proportion of 88 to 92 wt %, based on the total weight of theconductive silver paste.

Vanadium-Phosphorus-Antimony-Zinc-Based-Oxide

In one embodiment, the conductive silver paste comprisesvanadium-phosphorus-antimony-zinc-based-oxide. In one embodiment, thevanadium-phosphorus-antimony-zinc-based-oxide is a glass. In a furtherembodiment, the vanadium-phosphorus-antimony-zinc-based-oxide iscrystalline, partially crystalline, amorphous, partially amorphous, orcombinations thereof. The vanadium-phosphorus-antimony-zinc-based oxidecomprises V₂O₅, P₂O₅, Sb₂O₃ and ZnO and can be referred to asV—P—Sb—Zn—O. In an embodiment, thevanadium-phosphorus-antimony-zinc-based-oxide includes more than oneglass composition. In another embodiment, thevanadium-phosphorus-antimony-zinc-based-oxide includes a glasscomposition and an additional composition, such as a crystallinecomposition. The terms “glass”, “glass composition” or “glass frit” willbe used herein to represent any of the above combinations of amorphousand crystalline materials.

The glass compositions described herein may also include additionalcomponents as disclosed below.

The vanadium-phosphorus-antimony-zinc-based-oxide (V—P—Sb—Zn—O) may beprepared by mixing V₂O₅, P₂O₅, Sb₂O₃ and ZnO (or other materials thatdecompose into the desired oxides when heated) using techniquesunderstood by one of ordinary skill in the art. Such preparationtechniques may involve heating the mixture in air or anoxygen-containing atmosphere to form a melt, quenching the melt, andgrinding, milling, and/or screening the quenched material to provide apowder with the desired particle size. Melting the mixture of vanadium,phosphorus, antimony and zinc oxides is typically conducted to a peaktemperature of 800 to 1200° C. The molten mixture can be quenched, forexample, on a stainless steel platen or between counter-rotatingstainless steel rollers to form a platelet. The resulting platelet canbe milled to form a powder. Typically, the milled powder has a d₅₀ of0.1 to 3.0 microns. One skilled in the art of producing glass frit mayemploy alternative synthesis techniques such as but not limited to waterquenching, sol-gel, spray pyrolysis, quenching by splat cooling on ametal platen, or others appropriate for making powder forms of glass.

Glass compositions are described herein as including percentages ofcertain components. Specifically, the percentages are the percentages ofthe components used in the starting material that was subsequentlyprocessed as described herein to form a glass composition. Suchnomenclature is conventional to one of skill in the art. In other words,the composition contains certain components, and the percentages ofthose components are expressed as a percentage of the correspondingoxide form. The components of the glass composition may be supplied byvarious sources such as oxides, halides, carbonates, nitrates,phosphates, hydroxides, peroxides, halogen compounds and mixturesthereof. Herein, the composition of thevanadium-phosphorus-antimony-zinc-based-oxide is given in terms of theequivalent oxides no matter the source of the various components. Asrecognized by one of ordinary skill in the art in glass chemistry, acertain portion of volatile species may be released during the processof making the glass. An example of a volatile species is oxygen.

In one embodiment, the starting mixture used to make the V—P—Sb—Zn—O iscomprised of (based on the total weight of the V—P—Sb—Zn—O) 45-60 wt %V₂O₅, 15-30 wt % P₂O₅, 5-20 wt % Sb₂O₃ and 3-15 wt % ZnO. In anotherembodiment, the V—P—Sb—Zn—O is comprised of (based on the total weightof the V—P—Sb—Zn—O) 48-58 wt % V₂O₅, 18-28 wt % P₂O₅, 10-16 wt % Sb₂O₃and 6-12 wt % ZnO.

In a further embodiment, in addition to the above oxides, the startingmixture used to make the V—P—Sb—Zn—O may include small amounts of one ormore of TiO₂, Al₂O₃.SiO₂, SnO₂, and B₂O₃ or other components. In oneembodiment, the silver paste has a V—P—Sb—Zn—O further comprising 0.1-1wt % TiO₂ and 0.1-1 wt % Al₂O₃ (based on the total weight of theV—P—Sb—Zn—O).

In one embodiment, the V—P—Sb—Zn—O may be a homogenous powder. Inanother embodiment, the V—P—Sb—Zn—O may be a combination of more thanone powder, wherein each powder may separately be a homogenouspopulation. The composition of the overall combination of the twopowders is within the ranges described above. Separately, these powdersmay have different compositions, and may or may not be within the rangesdescribed above; however, the combination of these powders is within theranges described above.

If starting with a fired glass, one of ordinary skill in the art maycalculate the percentages of starting components described herein usingmethods known to one of skill in the art including, but not limited to:Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), InductivelyCoupled Plasma-Atomic Emission Spectroscopy (ICP-AES), and the like. Inaddition, the following exemplary techniques may be used: X-RayFluorescence spectroscopy (XRF); Nuclear Magnetic Resonance spectroscopy(NMR); Electron Paramagnetic Resonance spectroscopy (EPR); Mössbauerspectroscopy; electron microprobe Energy Dispersive Spectroscopy (EDS);electron microprobe Wavelength Dispersive Spectroscopy (WDS);Cathodo-Luminescence (CL).

One of ordinary skill in the art would recognize that the choice of rawmaterials could unintentionally include impurities that may beincorporated into the glass during processing. For example, theimpurities may be present in the range of hundreds to thousands ppm. Thepresence of the impurities would not alter the properties of the glass,the thick-film composition, or the fired device. For example, a solarcell containing the thick-film composition may have the efficiencydescribed herein, even if the thick-film composition includesimpurities.

The V—P—Sb—Zn—O is present in the conductive silver paste in aproportion of 0.2 to 3.0 wt %, based on the total weight of theconductive silver paste. In one embodiment, the V—P—Sb—Zn—O is presentin the conductive silver paste in a proportion of 0.5 to 2.0 wt %, basedon the total weight of the conductive silver paste.

The vanadium-phosphorus-antimony-zinc-based-oxide composition can beprepared by mixing and blending V₂O₅, Sb₂O₃ and ZnO powders, andoptionally, other components, The blended powder batch materials areloaded into a platinum alloy crucible and then inserted into a furnaceat 900-1000° C. using an air or O²-containing atmosphere. The durationof the heat treatment is 20 minutes following the attainment of a fullsolution of the constituents. The resulting low viscosity liquidresulting from the fusion of the constituents is then quenched by metalroller. The quenched glass is then milled and screened to provide apowder with a d₅₀ of 0.1 to 3.0 microns.

One embodiment made by the above process has a composition of 53.2 wt %V₂O₅, 23.3 wt % P₂O₅, 13.4 wt % Sb₂O₃, 9.0 wt % ZnO, 0.5 wt % TiO₂ and0.6 wt % Al₂O₃ (based on the total weight of the V—P—Sb—Zn—O). It wasprepared by mixing and blending the V₂O₅, P₂O₅, Sb₂O₃, ZnO, TiO₂ andAl₂O₃ powders and processing as described above.

Tellurium-Boron-Phosphorus-Based-Oxide

In another embodiment, the conductive silver paste comprisestellurium-boron-phosphorus-based-oxide. In one embodiment, thetellurium-boron-phosphorus-based-oxide is a glass. In a furtherembodiment, the tellurium-boron-phosphorus-based-oxide is crystalline,partially crystalline, amorphous, partially amorphous, or combinationsthereof. The tellurium-boron-phosphorus-based-oxide comprises TeO₂, B₂O₃and P₂O₅ and can be referred to as Te—B—P—O. In an embodiment, thetellurium-boron-phosphorus-based-oxide includes more than one glasscomposition. In another embodiment, thetellurium-boron-phosphorus-based-oxide includes a glass composition andan additional composition, such as a crystalline composition. The terms“glass”, “glass composition” or “glass frit” will be used herein torepresent any of the above combinations of amorphous and crystallinematerials.

The glass compositions described herein may also include additionalcomponents as disclosed below.

The tellurium-boron-phosphorus-based-oxide (Te—B—P—O) may be prepared bymixing TeO₂, B₂O₃ and P₂O₅ (or other materials that decompose into thedesired oxides when heated) using techniques understood by one ofordinary skill in the art. Such preparation techniques may involveheating the mixture in air or an oxygen-containing atmosphere to form amelt, quenching the melt, and grinding, milling, and/or screening thequenched material to provide a powder with the desired particle size.Melting the mixture of tellurium, boron and phosphorus oxides istypically conducted to a peak temperature of 800 to 1200° C. The moltenmixture can be quenched, for example, on a stainless steel platen orbetween counter-rotating stainless steel rollers to form a platelet. Theresulting platelet can be milled to form a powder. Typically, the milledpowder has a d₅₀ of 0.1 to 3.0 microns. One skilled in the art ofproducing glass frit may employ alternative synthesis techniques such asbut not limited to water quenching, sol-gel, spray pyrolysis, quenchingby splat cooling on a metal platen, or others appropriate for makingpowder forms of glass.

Glass compositions are described herein as including percentages ofcertain components. Specifically, the percentages are the percentages ofthe components used in the starting material that was subsequentlyprocessed as described herein to form a glass composition. Suchnomenclature is conventional to one of skill in the art. In other words,the composition contains certain components, and the percentages ofthose components are expressed as a percentage of the correspondingoxide form. The components of the glass composition may be supplied byvarious sources such as oxides, halides, carbonates, nitrates,phosphates, hydroxides, peroxides, halogen compounds and mixturesthereof. Herein, the composition of thetellurium-boron-phosphorus-based-oxide is given in terms of theequivalent oxides no matter the source of the various components. Asrecognized by one of ordinary skill in the art in glass chemistry, acertain portion of volatile species may be released during the processof making the glass. An example of a volatile species is oxygen.

In one embodiment, the starting mixture used to make the Te—B—P—O iscomprised of (based on the total weight of the Te—B—P—O) 80-95 wt %TeO₂, 1-10 wt % B₂O₃ and 1-10 wt % P₂O₅. In another embodiment, theTe—B—P—O is comprised of (based on the total weight of the Te—B—P—O)84-93 wt % TeO₂, 2-8 wt % B₂O₃ and 2-8 wt % P₂O₅.

In a further embodiment, in addition to the above oxides, the startingmixture used to make the Te—B—P—O may include small amounts of one ormore of TiO₂, Al₂O₃.SiO₂, SnO₂, and B₂O₃ or other components. In oneembodiment, the silver paste has a Te—B—P—O further comprising 0.1-5 wt% SnO₂ (based on the total weight of the Te—B—P—O).

In one embodiment, the Te—B—P—O may be a homogenous powder. In anotherembodiment, the Te—B—P—O may be a combination of more than one powder,wherein each powder may separately be a homogenous population. Thecomposition of the overall combination of the two powders is within theranges described above. Separately, these powders may have differentcompositions, and may or may not be within the ranges described above;however, the combination of these powders is within the ranges describedabove.

If starting with a fired glass, one of ordinary skill in the art maycalculate the percentages of starting components described herein usingmethods known to one of skill in the art including, but not limited to:Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), InductivelyCoupled Plasma-Atomic Emission Spectroscopy (ICP-AES), and the like. Inaddition, the following exemplary techniques may be used: X-RayFluorescence spectroscopy (XRF); Nuclear Magnetic Resonance spectroscopy(NMR); Electron Paramagnetic Resonance spectroscopy (EPR); Mössbauerspectroscopy; electron microprobe Energy Dispersive Spectroscopy (EDS);electron microprobe Wavelength Dispersive Spectroscopy (WDS);Cathodo-Luminescence (CL).

One of ordinary skill in the art would recognize that the choice of rawmaterials could unintentionally include impurities that may beincorporated into the glass during processing. For example, theimpurities may be present in the range of hundreds to thousands ppm, Thepresence of the impurities would not alter the properties of the glass,the thick-film composition, or the fired device. For example, a solarcell containing the thick-film composition may have the efficiencydescribed herein, even if the thick-film composition includesimpurities.

The Te—B—P—O is present in the conductive silver paste in a proportionof 0.2 to 3.0 wt %, based on the total weight of the conductive silverpaste. In one embodiment, the Te—B—P—O is present in the conductivesilver paste in a proportion of 0.5 to 2.0 wt %, based on the totalweight of the conductive silver paste.

The tellurium-boron-phosphorus-based-oxide composition can be preparedby mixing and blending TeO₂, B₂O₃ and P₂O₅ powders, and optionally,other components. The blended powder batch materials are loaded into aplatinum alloy crucible and then inserted into a furnace at 900-1000° C.using an air or O₂-containing atmosphere. The duration of the heattreatment is 20 minutes following the attainment of a full solution ofthe constituents. The resulting low viscosity liquid resulting from thefusion of the constituents is then quenched by metal roller. Thequenched glass is then milled and screened to provide a powder with ad₅₀ of 0.1 to 3.0 microns.

One embodiment made by the above process has a composition of 89.6 wt %TeO₂, 4.7 wt % B₂O₃, 4.7 wt % P₃O₅ and 1.0 wt % SnO₂ (based on the totalweight of the Te—B—P—O). It was prepared by mixing and blending theTeO₂, B₂O₃, P₂O₅ and SnO₂ powders and processing as described above.

Tellurium-Molybdenum-Cerium-Based-Oxide

In still another embodiment, the conductive silver paste comprisestellurium-molybdenum-cerium-based-oxide. In one embodiment, thetellurium-molybdenum-cerium-based-oxide is a glass. In a furtherembodiment, the tellurium-molybdenum-cerium-based-oxide is crystalline,partially crystalline, amorphous, partially amorphous, or combinationsthereof. The tellurium-molybdenum-cerium-based-oxide comprises TeO₂,MoO₃ and CeO₂ and can be referred to as Te—Mo—Ce—O. In an embodiment,the tellurium-molybdenum-cerium-based-oxide includes more than one glasscomposition. In another embodiment, thetellurium-molybdenum-cerium-based-oxide includes a glass composition andan additional composition, such as a crystalline composition. The terms“glass”, “glass composition” or “glass frit” will be used herein torepresent any of the above combinations of amorphous and crystallinematerials.

The glass compositions described herein may also include additionalcomponents as disclosed below.

The tellurium-molybdenum-cerium-based-oxide (Te—Mo—Ce—O) may be preparedby mixing TeO₂, MoO₃ and CeO₂ (or other materials that decompose intothe desired oxides when heated) using techniques understood by one ofordinary skill in the art. Such preparation techniques may involveheating the mixture in air or an oxygen-containing atmosphere to form amelt, quenching the melt, and grinding, milling, and/or screening thequenched material to provide a powder with the desired particle size.Melting the mixture of tellurium, molybdenum and cerium-oxides istypically conducted to a peak temperature of 800 to 1200° C. The moltenmixture can be quenched, for example, on a stainless steel platen orbetween counter-rotating stainless steel rollers to form a platelet. Theresulting platelet can be milled to form a powder. Typically, the milledpowder has a d₅₀ of 0.1 to 3.0 microns. One skilled in the art ofproducing glass frit may employ alternative synthesis techniques such asbut not limited to water quenching, sol-gel, spray pyrolysis, quenchingby splat cooling on a metal platen, or others appropriate for makingpowder forms of glass.

Glass compositions are described herein as including percentages ofcertain components. Specifically, the percentages are the percentages ofthe components used in the starting material that was subsequentlyprocessed as described herein to form a glass composition. Suchnomenclature is conventional to one of skill in the art. In other words,the composition contains certain components, and the percentages ofthose components are expressed as a percentage of the correspondingoxide form. The components of the glass composition may be supplied byvarious sources such as oxides, halides, carbonates, nitrates,phosphates, hydroxides, peroxides, halogen compounds and mixturesthereof. Herein, the composition of thetellurium-boron-phosphorus-based-oxide is given in terms of theequivalent oxides no matter the source of the various components. Asrecognized by one of ordinary skill in the art in glass chemistry, acertain portion of volatile species may be released during the processof making the glass. An example of a volatile species is oxygen.

In one embodiment, the starting mixture used to make the Te—Mo—Ce—O iscomprised of (based on the total weight of the Te—Mo—Ce—O) 45-65 wt %TeO₂, 20-35 wt % MoO₃ and 10-25 wt % CeO₂. In another embodiment, theTe—Mo—Ce—O is comprised of (based on the total weight of the Te—Mo—Ce—O)50-60 wt % TeO₂, 22-32 wt % MoO₃ and 12-20 wt % CeO₂.

In a further embodiment, in addition to the above oxides, the startingmixture used to make the Te—Mo—Ce—O may include small amounts of one ormore of TiO₂, Al₂O₃.SiO₂, SnO₂, and B₂O₃ or other components.

In one embodiment, the Te—Mo—Ce—O may be a homogenous powder. In anotherembodiment, the Te—Mo—Ce—O may be a combination of more than one powder,wherein each powder may separately be a homogenous population. Thecomposition of the overall combination of the two powders is within theranges described above. Separately, these powders may have differentcompositions, and may or may not be within the ranges described above;however, the combination of these powders is within the ranges describedabove.

If starting with a fired glass, one of ordinary skill in the art maycalculate the percentages of starting components described herein usingmethods known to one of skill in the art including, but not limited to:Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), InductivelyCoupled Plasma-Atomic Emission Spectroscopy (ICP-AES), and the like. Inaddition, the following exemplary techniques may be used: X-RayFluorescence spectroscopy (XRF); Nuclear Magnetic Resonance spectroscopy(NMR); Electron Paramagnetic Resonance spectroscopy (EPR); Mössbauerspectroscopy; electron microprobe Energy Dispersive Spectroscopy (EDS);electron microprobe Wavelength Dispersive Spectroscopy (WDS);Cathodo-Luminescence (CL).

One of ordinary skill in the art would recognize that the choice of rawmaterials could unintentionally include impurities that may beincorporated into the glass during processing. For example, theimpurities may be present in the range of hundreds to thousands ppm. Thepresence of the impurities would not alter the properties of the glass,the thick-film composition, or the fired device. For example, a solarcell containing the thick-film composition may have the efficiencydescribed herein, even if the thick-film composition includesimpurities.

The Te—Mo—Ce—O is present in the conductive silver paste in a proportionof 0.2 to 3.0 wt %, based on the total weight of the conductive silverpaste. In one embodiment, the Te—Mo—Ce—O is present in the conductivesilver paste in a proportion of 0.5 to 2.0 wt %, based on the totalweight of the conductive silver paste.

The tellurium-molybdenum-cerium-based-oxide composition can be preparedby mixing and blending TeO₂, MoO₃ and CeO₂ powders, and optionally,other components. The blended powder batch materials are loaded into aplatinum alloy crucible and then inserted into a furnace at 900-1000° C.using an air or O₂-containing atmosphere. The duration of the heattreatment is 20 minutes following the attainment of a full solution ofthe constituents. The resulting low viscosity liquid resulting from thefusion of the constituents is then quenched by metal roller. Thequenched glass is then milled and screened to provide a powder with ad₅₀ of 0.1 to 3.0 microns.

One embodiment made by the above process has a composition of 56.0 wt %TeO₂, 27.5 wt % MoO₃ and 16.5 wt % CeO₂ (based on the total weight ofthe Te—Mo—Ce—O). It was prepared by mixing and blending the TeO₂, MoO₃and CeO₂ powders and processing as described above.

Sintering Inhibitants

In one embodiment, the conductive silver paste further comprises asintering inhibitant that is dispersed in the organic vehicle. Thesintering inhibitant slows down sintering and is believed to therebyreduce shunting. In one such embodiment, the sintering inhibitant istitanium resinate or any compound that decomposes into titanium resinateat temperatures of 550° C. to 900° C. and mixtures thereof. The titaniumresinate is present in the conductive silver paste in a proportion of0.1 to 1 wt %, based on the total weight of the conductive silver paste.In one embodiment, the titanium resinate is present in the conductivesilver paste in a proportion of 0.1 to 0.7 wt %, based on the totalweight of the conductive silver paste. In another embodiment, thetitanium resinate is present in the conductive silver paste in aproportion of 0.2 to 0.4 wt %, based on the total weight of theconductive silver paste.

In an embodiment, the sintering inhibitant is titanium dioxide. Thetitanium dioxide is present in the conductive silver paste in aproportion of 0.1 to 1 wt %, based on the total weight of the conductivesilver paste.

The conductive silver paste may comprise one or more other inorganicadditives.

Organic Vehicle

The conductive silver paste comprises an organic vehicle. The organicvehicle is an organic solvent or an organic solvent mixture or, inanother embodiment, the organic vehicle is a solution of organic polymerin organic solvent.

A wide variety of inert viscous materials can be used as an organicvehicle. The organic vehicle is one in which the other constituents,i.e., the particulate conductive silver and either the V—P—O or theV—B—O are dispersible with an adequate degree of stability. Theproperties, in particular, the rheological properties, of the organicvehicle must be that they lend good application properties to theconductive silver paste composition, including: stable dispersion ofinsoluble solids, appropriate viscosity and thixotropy for application,appropriate wettability of the paste solids, a good drying rate, andgood firing properties.

The organic vehicle is typically a solution of one or more polymers inone or more solvents. The most frequently used polymer for this purposeis ethyl cellulose. Other examples of polymers are ethylhydroxyethylcellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and monobutyl ether of ethyleneglycol monoacetate. The most widely used solvents found in thick filmcompositions are ester alcohols and terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and high boiling alcohols and alcohol esters. In addition,volatile liquids for promoting rapid hardening after application on thesubstrate can be included in the vehicle. Various combinations of theseand other solvents are formulated to obtain the viscosity and volatilityrequirements desired.

The organic vehicle content in the conductive silver paste is dependenton the method of applying the paste and the kind of organic vehicleused. In one embodiment, it is from 5 to 20 wt %, based on the totalweight of the conductive silver paste composition. In anotherembodiment, it is from 9 to 15 wt %, based on the total weight of theconductive silver paste composition. These wt % include the organicsolvent, any organic polymer and any other organic additives.

The conductive silver paste may comprise one or more other organicadditives, for example, surfactants, thickeners, rheology modifiers andstabilizers. An organic additive may be part of the organic vehicle.However, it is also possible to add an organic additive separately whenpreparing the conductive silver paste.

Conductive Silver Paste

The application viscosity of the conductive silver paste is in the rangeof 150 to 300 Pa·s when it is measured at a spindle speed of 10 rpm and25° C. by a utility cup using a Brookfield HBT viscometer and #14spindle.

The conductive silver paste is applied to the holes of the silicon waferto provide metallization and a conducting via from the front-side to theback-side of the metal-wrap-through solar cell, or from the backside tothe front side. The conductive silver paste is applied in a way tocompletely fill the hole with conductive silver or in the form of alayer to cover at least the inside of the holes with a metallization,i.e. to form the metallization of at least the inside of the holes.

The method of conductive silver paste application may be printing, forexample, screen printing. The application may be performed from thefront-side and/or from the back-side of the solar cell.

After application, the conductive silver paste is dried, for example,for a period of 1 to 10 minutes with the silicon wafer reaching a peaktemperature in the range of 100° C. to 399° C. Drying can be carried outmaking use of, for example, belt, rotary or stationary driers and inparticular, IR (infrared) belt driers.

The dried conductive silver paste is fired to form the finishedmetallization of the holes. These metallization serve as emittercontacts and back-side contacts of the MWT silicon solar cell. Thefiring is performed for a period of 1 to 5 minutes with the siliconwafer reaching a peak temperature in the range of 550° C. to 900° C. Thefiring can be carried out making use of single or multi-zone beltfurnaces, in particular, multi-zone IR belt furnaces. The firing cantake place in an inert gas atmosphere or in the presence of oxygen,e.g., in the presence of air. During firing the organic substanceincluding non-volatile organic material and the organic portion notevaporated during the drying is removed. The organic substance removedduring firing includes organic solvent, organic polymer and any organicadditives present.

The conductive silver paste firing process can be a co-firing process inwhich front-side metallization in the form of thin conductive metalcollector lines arranged in a pattern typical for MWT silicon solarcells and applied from a conductive metal paste and/or silver backsidecollector contacts applied from a back-side silver paste are fired atthe same time.

The conductive silver paste can be applied to MWT silicon solar cellsthat have emitters within the vias as well as to MWT silicon solar cellsthat do not have emitters within the vias. The conductive silver pastecan also be applied to MWT silicon solar cells that have antireflectivecoating within the vias as well as to MWT silicon solar cells that donot have antireflective coating within the vias. The conductive silverpaste can be applied to MWT silicon solar cells with n-type or p-typesilicon bases.

Also provided is a metal-wrap-through silicon solar cell comprising thefired conductive silver paste of the invention.

What is claimed is:
 1. A conductive silver paste comprising: (a) silver;(b) a vanadium-phosphorus-antimony-zinc-based-oxide comprising 45-60 wt% V₂O₅, 15-30 wt % P₂O₅, 5-20 wt % Sb₂O₃ and 3-15 wt % ZnO, wherein thewt % are based on the total weight of thevanadium-phosphorus-antimony-zinc-based-oxide; and (c) an organicvehicle, wherein the silver and thevanadium-phosphorus-antimony-zinc-based-oxide are dispersed in theorganic vehicle.
 2. The conductive silver paste of claim 1, saidvanadium-phosphorus-antimony-zinc-based-oxide comprising 48-58 wt %V₂O₅, 18-28 wt % P₂O₅, 10-16 wt % Sb₂O₃ and 6-12 wt % ZnO, wherein thewt % are based on the total weight of thevanadium-phosphorus-antimony-zinc-based-oxide.
 3. The conductive silverpaste of claim 1, wherein said silver is selected from the groupconsisting of silver in the form of spherical silver particles having ad₅₀ of from 1.7 to 1.9 μm and a d₁₀≧1 μm and a d₉₀≦3.8 μm, silver in theform of irregular particles having a d₅₀ of from 5.4 to 11.0 μm and ad₉₀ of from 9.6 to 21.7 μm and mixtures thereof.
 4. The conductivesilver paste of claim 1, saidvanadium-phosphorus-antimony-zinc-based-oxide further comprising 0.1-1wt % TiO₂ and 0.1-1 wt % Al₂O₃, wherein the wt % are based on the totalweight of said vanadium-phosphorus-antimony-zinc-based-oxide.
 5. Theconductive silver paste of claim 1, said conductive silver pastecomprising 85-95 wt % silver and 0.2 to 3.0 wt %vanadium-phosphorus-antimony-zinc-based-oxide, based on the total weightof said conductive silver paste.
 6. The conductive silver paste of claim1, said conductive silver paste further comprising a 0.1 to 1 wt %sintering inhibitant selected from the group consisting of titaniumresinate and titanium dioxide, wherein the wt % are based on the totalweight of the conductive silver paste.
 7. A metal-wrap-through siliconsolar cell with an n-type or a p-type silicon base comprising the firedconductive silver paste of claim
 1. 8. A conductive silver pastecomprising: (a) silver; (b) a tellurium-boron-phosphorus-based-oxidecomprising 80-95 wt % TeO₂, 1-10 wt % B₂O₃ and 1-10 wt % P₂O₅, whereinthe wt % are based on the total weight of thetellurium-boron-phosphorus-based-oxide; and (c) an organic vehicle,wherein the silver and the tellurium-boron-phosphorus-based-oxide aredispersed in the organic vehicle.
 9. The conductive silver paste ofclaim 8, said tellurium-boron-phosphorus-based-oxide comprising 84-93 wt% TeO₂, 2-8 wt % B₂O₃ and 2-8 wt % P₂O₅, wherein the wt % are based onthe total weight of the tellurium-boron-phosphorus-based-oxide.
 10. Theconductive silver paste of claim 8, wherein said silver is selected fromthe group consisting of silver in the form of spherical silver particleshaving a d₅₀ of from 1.7 to 1.9 μm and a d₁₀≧1 μm and a d₉₀≦3.8 μm,silver in the form of irregular particles having a d₅₀ of from 5.4 to11.0 μm and a d₉₀ of from 9.6 to 21.7 μm and mixtures thereof.
 11. Theconductive silver paste of claim 8, saidtellurium-boron-phosphorus-based-oxide further comprising 0.1-5 wt %SnO₂, wherein the wt % is based on the total weight of saidtellurium-boron-phosphorus-based-oxide.
 12. The conductive silver pasteof claim 8, said conductive silver paste comprising 85-95 wt % silverand 0.2 to 3.0 wt % tellurium-boron-phosphorus-based-oxide, based on thetotal weight of said conductive silver paste.
 13. The conductive silverpaste of claim 8, said conductive silver paste further comprising a 0.1to 1 wt % sintering inhibitant selected from the group consisting oftitanium resinate and titanium dioxide, wherein the wt % are based onthe total weight of the conductive silver paste.
 14. Ametal-wrap-through silicon solar cell with an n-type or a p-type siliconbase comprising the fired conductive silver paste of claim
 8. 15. Aconductive silver paste comprising: (a) silver; (b) atellurium-molybdenum-cerium-based-oxide comprising 45-65 wt % TeO₂,20-35 wt % MoO₃ and 10-25 wt % CeO₂, wherein the wt % are based on thetotal weight of the tellurium-molybdenum-cerium-based-oxide; and (c) anorganic vehicle, wherein the silver and thetellurium-molybdenum-cerium-based-oxide are dispersed in the organicvehicle.
 16. The conductive silver paste of claim 15, saidtellurium-molybdenum-cerium-based-oxide comprising 50-60 wt % TeO₂,22-32 wt % MoO₃ and 12-20 wt % CeO₂, wherein the wt % are based on thetotal weight of the tellurium-molybdenum-cerium-based-oxide.
 17. Theconductive silver paste of claim 15, wherein said silver is selectedfrom the group consisting of silver in the form of spherical silverparticles having a d₅₀ of from 1.7 to 1.9 μm and a d₁₀≧1 μm and ad₉₀≦3.8 μm, silver in the form of irregular particles having a d₅₀ offrom 5.4 to 11.0 μm and a d₉₀ of from 9.6 to 21.7 μm and mixturesthereof.
 18. The conductive silver paste of claim 15, said conductivesilver paste comprising 85-95 wt % silver and 0.2 to 3.0 wt %tellurium-molybdenum-cerium-based-oxide, based on the total weight ofsaid conductive silver paste.
 19. The conductive silver paste of claim15, said conductive silver paste further comprising a 0.1 to 1 wt %sintering inhibitant selected from the group consisting of titaniumresinate and titanium dioxide, wherein the wt % are based on the totalweight of the conductive silver paste.
 20. A metal-wrap-through siliconsolar cell with an n-type or a p-type silicon base comprising the firedconductive silver paste of claim 15.